The present invention relates generally to the field of semiconductor device manufacturing, and more specifically to a method and apparatus for lithographically printing tightly nested device features and isolated device features in integrated circuits.
In the semiconductor industry, there is a continuing effort to produce devices having a higher density of device features. As a result, the printing of device feature patterns with extremely small spacings has been and continues to be required. However, the printing of such tightly nested features or contacts presents problems associated with the process window for the lithography process. The process window is a measure of the amount of process variation that can be tolerated while still maintaining the printed feature sizes within set tolerances. Common measurements of process window include depth of focus (DOF), exposure latitude (EL), and total process window (TW). TW is a measure of the area under the curve in a plot of DOF vs. EL.
When printing relatively isolated small contacts, a sufficient process window can be achieved through the use of attenuated phase-shifting masks along with a reduced partial coherence factor. Partial coherence factor is the ratio of the illumination pupil size to the imaging pupil size, and is a measure of the coherency of the imaging system. As the partial coherence factor approaches zero, the degree of coherence increases. As the coherency of an imaging system increases (and the partial coherence factor decreases), the interaction between neighboring features in an image increases. Thus, with an imaging system having a high degree of coherence, the way a feature is imaged, i.e., its size and shape when printed, depends on the other features surrounding it. With an imaging system having a low degree of coherence, the effect of the size and shape of each feature on its neighbor is much less than that of a coherent system.
In a photolithography process in which a mask pattern is imaged onto a semiconductor substrate, the degree of coherence has additional implications. An imaging system having a high degree of coherence allows stronger phase interactions to occur. In such a system, interaction will be greater between light passing through different points of the mask being imaged when projected onto the semiconductor substrate. Therefore, by carefully controlling the phase of the light passing through regions of the mask surrounding a feature, the way that feature will be imaged can be modified. This phase interaction will not occur with an imaging system having a low degree of coherence, and therefore changing the phase on regions of the mask surrounding the feature will not impact the way that feature is imaged. A mask which allows the phase of the light passing through various regions to be adjusted is called a phase-shifting mask. Thus, an imaging system having a high degree of coherence generally improves the performance of phase shifting masks used in printing relatively isolated device features.
However, tightly nested small contacts cannot be resolved using attenuated phase-shifting masks and reduced partial coherence factor alone. FIGS. 1A and 1B illustrate the problem of reduced process window associated with tightly spaced device features. FIG. 1A illustrates TW as a function of x pitch and y pitch, and FIG. 1B illustrates DOF at 10% EL as a function of x pitch and y pitch. The pitch is the distance from one feature to the next adjacent feature in either the x direction or y direction as shown in FIG. 3A. In both FIGS. 1A and 1B, the partial coherence factor ("sgr") is set to a relatively low value of 0.45. FIGS. 1A and 1B show that as the pitch is reduced in either the x direction or y direction, or both, the process window decreases rapidly. For example, in FIG. 1A, when the x pitch and y pitch are both a relatively large 740 nm, TW is about 28%-xcexcm, which is acceptable for present-day processes. When the x pitch is reduced to 400 nm, leaving the y pitch at 740 nm, TW drops to about 14%-xcexcm. Likewise, when the y pitch is reduced to 400 nm, leaving the x pitch at 740 nm, TW also drops to about 14%-xcexcm. When both the x pitch and y pitch are reduced to 400 nm, TW drops even further to about 10%-xcexcm, which is unacceptable for present-day processes.
This phenomenon is believed to be caused by diffraction of the light. As the pitch is reduced, the diffracted light is believed to spread wider at the pupil plane of the imaging lens. If the pitch becomes too small, a limit is reached where only one diffracted order falls within the lens pupil, resulting in no modulation of the light at the wafer plane and the mask pattern being completely unresolved. This limit or cutoff occurs when
pitch=xcex/(NA*(1+"sgr"))
where xcex is the imaging wavelength, NA is the numerical aperture (a measurement of the size of the imaging pupil), and "sgr" is the partial coherence factor.
A known solution to improving the process window in the printing of features having a tight pitch is to increase the partial coherence factor and thereby reduce the minimum resolvable pitch. In FIGS. 2A and 2B, the partial coherence factor ("sgr") has been increased to 0.75. FIG. 2A illustrates TW as a function of x pitch and y pitch, and FIG. 2B illustrates DOF at 10% EL as a function of x pitch and y pitch. FIGS. 2A and 2B each show that when a contact is nested more tightly in any direction, i.e. as the pitch is reduced in any direction, the process window to print the contact remains acceptable. Note that there is no rapid decrease in process window as either the x pitch or y pitch, or both, decreases. However, at this relatively high partial coherence factor, the resolution in printing isolated small features suffers due to a reduction in phase interaction. Thus, when printing a mask pattern having both tightly nested and isolated device features, lithographers are often required to compromise between a sufficient process window for one and good imaging resolution for the other.
One solution to this dilemma was proposed in U.S. Pat. No. 5,424,154 to Borodovsky. Borodovsky discloses a method of improving lithographic resolution for isolated features on a mask which also contains tightly nested features. In this method, complementary or xe2x80x9cdummyxe2x80x9d features are added to isolated device features on a first mask so that the pitch of the isolated features is reduced to approximately the same as that of the tightly nested features. The dummy features are then obliterated by exposure to a second mask.
A similar solution was proposed in U.S. Pat. Nos. 5,242,770 and 5,447,810, both to Chen et al. Chen discloses methods of reducing proximity effects when printing both isolated features and tightly nested features on a mask. The process window of isolated features is improved by adding additional or xe2x80x9cdummyxe2x80x9d features adjacent to isolated features edges in the mask. The dummy features are the same transparency as the original feature and have dimensions less than the resolution of the exposure tool. Therefore, these dummy features are not transferred onto the photoresist layer.
Thus, in both the Borodovsky and Chen methods, dummy features must be added to the mask pattern to convert isolated features into tightly nested features. In addition, the Borodovsky method requires a second exposure to remove the dummy features created with the first exposure.
Another solution to the dilemma between sufficient process window and good imaging resolution was proposed in U.S. Pat. No. 5,563,012 to Neisser. Neisser describes methods of splitting a mask pattern having both isolated and tightly nested features into two or more modified or overlay masks. The mask features are divided among the two or more overlay masks such that each mask contains features having the same pitch. In a first embodiment, a mask pattern is split into two or more overlay masks, each having features which are relatively isolated, i.e., which have a relatively large pitch. The tightly nested features are divided into two or more overlay masks so that the resulting pitch of these features is approximately the same as the pitch of the isolated features. The isolated features are then added to any one of the overlay masks. Each overlay mask pattern is then lithographically printed onto a substrate using the same exposure conditions for each mask. In a second embodiment, a mask pattern is split into two or more overlay masks, each having features which are tightly nested. The isolated features of the mask pattern are added to one or more of the overlay masks, along with xe2x80x9cdummyxe2x80x9d features, so that the resulting pitch of these features is approximately the same as the tightly nested features. The tightly nested features are then added to any one of the overlay masks. Each overlay mask pattern is then printed onto a substrate, again using the same exposure conditions for each mask.
Two observations can be made regarding the Neisser methods. In the second Neisser embodiment, as in the Borodovsky and Chen methods, dummy features must be added to the mask pattern. In the first Neisser embodiment, dummy features are not required. However, in this first embodiment, more than two overlay masks may be required under certain conditions. In a mask pattern having isolated features and an array of tightly nested features, splitting the array of tightly nested features into only two overlay mask patterns may not result in a pitch sufficiently large to match that of the isolated features. The array may need to be split into three or more mask patterns to achieve a large enough pitch. For example, an array of three tightly nested features in a xe2x80x9cTxe2x80x9d or triangular formation can not be split into less than three mask patterns in order to achieve the goal of reducing the pitch of these features. As another example, in a mask pattern having a square array of tightly nested features with a pitch 2xc3x97 that of the feature size and having isolated features with a pitch of 4xc3x97 that of the feature size, splitting the array of tightly nested features into only two mask patterns will result in a pitch of at most 2.8xc3x97 that of the feature size. (2.8 is the length of the diagonals in an array of squares having sides of length 2.) This will result in at least one of the two overlay masks having isolated features with a pitch of 4xc3x97 and a portion of the tightly nested features having a pitch of 2.8xc3x97, which may not be a sufficiently close pitch in order to optimize both the process window and imaging resolution.
Therefore, there is a need in the art for a method of printing a mask pattern having tightly nested and isolated device features, wherein both the process window and imaging resolution can be optimized for each category of features. Further, there is a need in the art for a method of successfully printing a mask pattern having tightly nested and isolated features, which does not require the addition of dummy features to the mask pattern, and which allows flexibility in the number of masks required.
The present invention solves the problem of optimizing both the process window and imaging resolution for tightly nested and isolated features in a mask pattern, and does so without requiring the addition of dummy features to the mask pattern, while maintaining flexibility in the number of masks required.
In one aspect of the present invention, a method of lithographically printing a pattern on a substrate is disclosed, wherein the pattern includes features with diverse pitches. This method comprises the steps of: (1) grouping the features into a plurality of feature groups according to pitch, said plurality of feature groups including at least one feature group wherein the pitch is less than at least one predetermined value and at least one feature group wherein the pitch is greater than at least one predetermined value; (2) forming a plurality of masks, each mask including a different one of said plurality of feature groups; (3) depositing at least one photosensitive layer on the substrate; (4) successively positioning each of the plurality of masks above the substrate; and (5) successively exposing each of the plurality of masks on said at least one photosensitive layer.
In another aspect of the present invention, a method of forming a plurality of lithography masks from a single lithography mask pattern having features with diverse pitches is disclosed. This method comprises the steps of: (a) comparing the pitch of each feature to at least one predetermined value; (b) adding each feature to one of a plurality of feature groups according to pitch, said plurality of feature groups including at least one feature group wherein the pitch is less than said at least one predetermined value and at least one feature group wherein the pitch is greater than said at least one predetermined value; and (c) forming a plurality of lithography masks, each mask including a different one of said plurality of feature groups.
In yet another aspect of the present invention, an apparatus for lithographically printing a mask pattern having features with diverse pitches is disclosed. The apparatus comprises a plurality of lithography masks formed by a method comprising the steps of: (a) comparing the pitch of each feature to at least one predetermined value; (b) adding each feature to one of a plurality of feature groups according to pitch, said plurality of feature groups including at least one feature group wherein the pitch is less than said at least one predetermined value and at least one feature group wherein the pitch is greater than said at least one predetermined value; and (c) forming a plurality of lithography masks, each mask including a different one of said plurality of feature groups.